The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method for reducing image artifacts caused by flow, motion and phase aliasing.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .delta. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.1, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence, the magnitude of the net transverse magnetic moment M.sub.1 depends primarily on the length of time and magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited nuclei induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.1. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.o e.sup.t/t* 2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements." Such NMR measurements are divided into a period of excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more excitation pulses (B.sub.1) of varying magnitude and duration. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.1 over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is subjected to a sequence of NMR measurement cycles which vary according to the particular localization method being used. The received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the NMR signals can be identified.
NMR has rapidly developed into an imaging modality which is utilized to obtain tomographic, projection and volumetric images of anatomical features of live human subjects. Such images depict the nuclear-spin distribution (typically protons associated with water and fat), modified by specific NMR properties of tissues, such as spin lattice (T.sub.1), and spin-spin (T.sub.2) relaxation time constants. They are of medical diagnostic value because they depict anatomy and allow tissue characterization. However, such magnetic resonance images are often plagued by artifacts caused by fluid flow (flow artifacts) and by body motion (motion artifacts).
Referring particularly to FIG. 8, an NMR system employs a receiver coil which is receptive to NMR signals produced anywhere within a relatively large "receptive volume." In a whole body NMR system, for example, the receiver coil is sensitive to NMR signals produced in a circular cylindrical receptive volume which is coextensive with the lumen in which the patient is positioned. Within this receptive volume, an image volume is defined by the magnetic field gradients, excitation pulse frequency bandwidth and receiver frequency bandwidth. While this image volume is smaller than the receptive volume, it is often larger than the particular area, or volume which is of clinical interest. It is the structures within the clinical volume of interest which are of interest to the clinician and which must be imaged accurately and clearly.
Such structures are represented in FIG. 8 by the object "A". Unfortunately, events which occur outside the clinical volume of interest can produce artifacts which obscure object A. For example, object "B", which lies in the image volume, may be in motion due to respiration, and it can produce motion artifacts which spill into the clinical volume of interest and obscure the object A. Similarly, objects which are in the receptive volume, but which lie outside the image volume, may produce artifacts inside the clinical volume of interest. The object "C", for example, may be aliased into the volume of interest where it obscures the object A. And finally, there may be objects "D" which lie outside the clinical volume of interest along the z axis (the direction of the polarizing magnetic field) which either move into the volume of interest during the NMR scan or which move in and out during the scan. This occurs, for example, as a result of blood flowing into the volume of interest or objects moving in response to respiration. In either case, fresh spins are introduced which distort the image.
Several kinds of flow artifacts can cause problems in clinical NMR imaging. In FIG. 9a, blood vessels (a) are shown passing through the plane-of-section of an NMR image (b). The ideal NMR image which is reconstructed is shown in FIG. 9b and it accurately depicts vascular structures with a high degree of contrast between lumens and surrounding tissue. Vascular structures tend to have low signal intensity with most NMR pulse sequences employed in clinical application. This is due to a number of physical mechanisms, including washout and dephasing of excited spins during the NMR measurement cycle. In an accurate image, the low intensity of flowing blood enables identification of intraluminal pathology such as intimal flaps (c) associated with atherosclerotic dissection and thrombi or atherosclerotic plaques (d), both of which appear clearly in FIG. 9b.
As shown in FIG. 9c artifacts that are created during the NMR reconstruction can severely degrade the diagnostic quality of images by obscuring anatomic detail. A number of physical mechanisms such as flow related enhancement and even-echo rephasing of excited spins can lead to increased intensity within vascular structures. The view-to-view phase and amplitude variations of intraluminal contents can cause severe artifacts that extend into the image around vessels (e). The pattern of flow artifacts depends on the characteristics of flow, the type of NMR sequence, and the reconstruction technique. The presence of increased intensity within vascular lumens can easily mask pathological findings and prevent their recognition (f). In some cases, the intraluminal signals can simulate the appearance of pathological structures such as thrombi. The process also tends to decrease the contrast between vascular structures and surrounding tissues (g), thereby reducing the morphological information in the image. An admixture of these phenomena affects most vessels in typical NMR images.
A number of methods have been proposed for eliminating flow and motion artifacts. Gating techniques are employed, for example, to synchronize the NMR scan with the respiratory or cardiac cycle in order to minimize motion artifacts. Several NMR pulse sequences have been proposed to either desensitize the NMR measurement to the phase perturbations caused by flowing spins, or to sensitize it to flow in such a manner that the effects of flow can be separated from the reconstructed images. None of these methods have proven entirely satisfactory, either from a performance standpoint, or because of their adverse impact on scan time or the type of NMR measurements that maY be performed.